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Reducing voltage for a driver driven LED strip

Discussion in 'LEDs and Optoelectronics' started by john2k, Sep 21, 2019.

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  1. john2k


    Jun 13, 2012
    I have the following glow strip wire which comes with a 12V driver that you plug directly into a 12v source from the car. However, directly plugged in the drivers makes a humming sound and I dont want to hear this at all. I did a bench test with a variable voltage PSU and I much prefer the light output when I put the voltage at 3.3V. Because its for ambient lighting I feel the full brightness is too bright for nighttime. Is there a inline resistor I can use on the positive wire that wont generate too much heat that I can heatshrink on?

  2. Audioguru


    Sep 24, 2016
    Is the humming produced by a bee in the little box connected to the red and black wires? Do you need the little box?

    The value for a resistor is calculated from the current you want when your supply was 3.3V and the voltage that will be across the resistor.

    The power wasted by the resistor (making heat) is calculated from the voltage across it multiplied by the current in it.
  3. dave9


    Mar 5, 2017
    Yes, a series resistor is one way you can do it. Measure the current when it is running at the desired brightness, in this case at 3.3V if you've settled on that, then plug the numbers into an LED calculator. You should also decide whether you want that target brightness at the vehicle (alternator) running state voltage, which you could assume is around 14.4V or measure it, then have it a little dimmer at the vehicle off state voltage of 12.6V (assuming 12V battery), or vice versa.

    So if you want the brightness fixed at 14.4V the site above would have:

    Supply Voltage 14.4
    Forward Voltage 3.3
    Forward Current (in mA) - whatever you measure with your 3.3V bench test, in this example I input 150 (mA).
    Resistor Value 74 ohms (round up or down if needed to arrive at a popular value)

    Power 1.665W - This is close enough to 2W, that I would move up to a 3W resistor instead of 2W to make the thermal density lower. It will get hot.

    A buck current regulating switching PSU module would be more efficient, and are only $2-3 on ebay (with a month wait for shipping from China) but if talking about only a couple watts loss, then a resistor would be acceptable to me. I would consider where you put it since, again it will get hot.

    Are you going to leave the resistor unmounted or mount it? Heatshrink tubing might not last long if that's the only thing you depend on to insulate it thermally and electrically from its surroundings, so if you have large diameter heatshrink tubing to fit the whole resistor inside (if you're not mounting it) you could just go with a higher wattage rated resistor, or you could split up (spread out over a larger area) the heat using multiple resistors in series or parallel to reach the resistance value you need.

    If you're mounting it, you could get a resistor that's heatsink-able and mount it to chassis metal somewhere which will heatsink it some and by its sheer size (and corresponding overkill wattage rating) will run cooler. The heatshrink tubing would then only cover the wire and terminals of the resistor, not the whole body of it. Here are some examples:

    Remember that much of what I wrote above, depends on that 150 mA value I randomly choose to input into the Digikey calculator to result in 1.665W. Do the calculation using your own numbers to see how much resistor wattage you will have and go from there.

    It is always good to add a fuse to vehicle add-on circuits, where you make the new tap into the existing electrical system rather than at the load so everything added has that protection. You may already know this, but a random site visitor might not think about it.

    Edit: One other thing to keep in mind is that if you are using multiple glow strips, connecting them in series will mean a higher forward voltage and so a lower voltage drop from 14.4V, so you could use a lower ohm resistor that has lower wattage and heat dissipation. Just use the sum of their forward voltages which in the example I posed above would be (for two strips in series, 2 x 3.3V = ) 6.6V as the forward voltage in the LED calculator.
    Last edited: Sep 21, 2019
    john2k likes this.
  4. john2k


    Jun 13, 2012
    So I bought a 2W 180ohm resistance metal film resistor to test it. It works well and is almost at the level where the buzzing sound is not there and I left it plugged in to a car battery directly for over 1.5hrs and the resistor did not get hot at all. I feel Iike if I take it a notch further then I can completely get rid of the buzzing. If I wanted to do that should I try 2W 200ohms or keep ohms the same and increase the watts?
  5. dave9


    Mar 5, 2017
    The buzzing sound is probably a resonant frequency of the inductor windings in the driver. I can't predict whether a little more or less current will help but it could. You'd change the resistance not the wattage.

    Another thing you can try if the driver isn't potted is open it up and lather the inductor with epoxy or cement to fix the coils in place.
    john2k likes this.
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